1. Field of the Invention
This invention relates to a novel photosensitive composition photopolymerizable by the action of actinic light. More particularly, this invention relates to photosensitive compositions comprising an unsaturated polyester possessing unsaturation in both the acid and alcohol portion, a methylolated phenolic crosslinking agent, and a metal halide photoinitiator. This invention further relates to gasketing and glass fiber impregnates utilizing this photosensitive composition.
2. Description of the Prior Art
It is recognized that polymeric coatings, impregnates and mold compositions, to receive wide acceptance, should respond readily to readily available crosslinking agents such as of the peroxide, sulfurvulcanizing or phenolic type. These compositions should not exhibit reversion tendencies at cure temperatures, and, when cured, should exhibit a suitable balance of properties such as elongation, compression set, heat stability, water resistance, oil, fuel and solvent resistance, and low temperature flexibility. Typically, the prior art impregnates, coatings and mold compositions are generally of an unsaturated nature and, in order to be cured, are subject to thermal treatment at relatively high temperatures, e.g. over 180.degree. C., so as to establish a final crosslinked composition that is of acceptable physical properties. These compositions, while finding great commercial use, are energetically detrimental in view of the high temperatures necessary to initiate the curing. Further, substrates that are sensitive to thermal decomposition cannot be used with prior art compositions of this nature.
In order to obviate the problems of the high-temperature curing in the elastomeric polyesters, photoirradiation curing techniques have become popular. Namely, monomers, oligomers, and prepolymers possessing polymerizable unsaturation can advantageously be photopolymerized by the use of suitable photopolymerizable crosslinking agents and initiators. The crosslinking agents of particular use in the prior art are those polythiol materials having a wide range of molecular weight, such being active either in the presence of oxygen or under inert conditions, and utilizing photochemical initiators. For example, U.S. Pat. No. 3,908,039 discloses such thiol crosslinking system that is used in combination with a polyene resin component formed from a polybasic acid or anhydride and a polyhydric alcohol, such end capped with allylic or vinyl unsaturation. The photoinitiators described in this patent are a variety of the organic initiators such as benzophenone and the like. While the prior art thiol crosslinked polyester unsaturates provide cured compositions having acceptable properties, the cost of such system is relatively high, limiting their usefulness.
It is known that metallic salts are useful in thermally reactive resins such being further cured by the action by actinic light. Chrome-containing polyvinyl alcohols and polyvinyl alcohol cinnamate, for example, can be photocured in this manner to give a thin, photocrosslinked layer. The articles prepared by such photopolymerization, however, are known to suffer from insufficient hardness, strength and solvent resistance.
The prior art impregnates and coatings referred to above have found a variety of ultimate uses, mostly dependent on the final desired properties of the formed composites. The use of these crosslinkable compositions as impregnating agents for forming gasketing materials is widely recognized.
Gaskets are made by first preparing a slurry of asbestos or cellulose fiber in water. A charge, opposite to that of the crosslinkable impregnate composition (in the form of a latex) is placed on the fiber surface and the curing agents incorporated. The crosslinkable latex is then introduced and stirring continued until an aliquot shows clear liquor, that is, until the crosslinkable impregnate has been deposited on the fiber. The whole is then fed onto the Fourdrinier, drained, possibly calendered, and cured.
Recent government regulations pertaining to asbestos have led to attention being turned to glass papers being used to replace the asbestos-containing sheets, not only for gaskets but also for surface covering backings.
The formed mats are, however, low in breaking strength and poor in the MIT fold test. They also are extremely porous, having little air flow resistance.